In order to protect the lives of passengers during a traffic accident, modern vehicles are generally fitted with a protection system comprising several airbags and seat belt pretensioners, which are used to absorb the energy of a passenger released during the collision due to the accident. It is clear that such systems are most effective when they are well adapted to the specific requirements of each passenger, i.e. to the weight and/or the size of the passenger. That is why microprocessor-controlled protection systems have been designed which provide several operational modes, for example allowing an adaptation of the instant at which airbags are deployed and their volume, of the instant at which safety belts are released after the collision, etc., as a function of the build of the passenger and the position of the passenger on the seat.
In order to enable the control microprocessor to select the optimum operational mode for a given passenger, it is therefore necessary to have available a method and a device for detecting the build or bodily form of the passenger which determines the size and/or the weight and/or the position of the passenger and which indicates this to the control circuit of the protection system.
For this purpose, the patent U.S. Pat. No. 5,232,243 describes a device for detecting the weight of a passenger which comprises several individual force sensors arranged in a matrix array in the vehicle seat cushion. The force sensors have an electric resistance that varies with the applied force and are known by the abbreviation FSR (force sensing resistor). The resistance of each sensor is measured individually and, by adding the forces corresponding to the values of these resistances, an indication is obtained of the total force exerted, i.e. of the weight of the passenger. In other words, the method used in U.S. Pat. No. 5,232,243 consists of directly associating a specific weight to a specific reading of the sensor.
However, the total weight of a passenger does not act solely on the surface of the seat, since part of the weight is supported by the passenger's legs, which rest on the bottom of the vehicle, and another part rests on the back of the seat. In addition, the ratios between the various parts vary considerably with the passenger's position on the seat, which causes the total force measured by the individual force sensors not to correspond to the real weight of the passenger but to experience very large variations depending on the passenger's posture on the seat. This means on the other hand, that the same reading of the sensor, i.e. the same distribution of individually measured forces in the case of a sensor comprising individual force sensors, can be caused by the presence of passengers having rather different physical properties. Hence there is a risk of wrong classification of a specific passenger, which will cause the restraint system to be deployed in a non-adapted mode.